Screw pump belongs to high efficiency oil-extracting apparatus, there are two main reasons for the shutdown and reversal of motor of the screw pump:
1. When the rotator is made to rotate by the drive device of rod pump, elastic deformation will be produced in the long polished rod. The downhole rotator starts rotate after the most apex of polished rod rotates for tens of cycles, the elastic energy of these tens of cycles will be released when shutdowns, thus the parts of the polished rod close to ground will reversely rotate back with high speed if no inhibition is applied, the high speed reverse rotation will lead to reversal tripping accident of polished rod because the polished rod is formed by many nodes connected to each other (except continuous rod). To ensure the safety of polished rod, braking means must be employed.
2. When shut downs, the reflux of liquid will be produced by the action of liquid pressure and pipeline pressure in the tubing and will drive the rotator of screw pump to rotate reversely with high speed like a hydro-generator, such phenomena will happen in both ground drive rod pump and rodless pump submerged in the oil downhole. When liquid flows reversely, the part of the transmission shaft connected to the rotator will always be broken by the drastic liquid reflux (especially for submerged rodless pump).
At present, there are two main braking means are employed by screw pump: the first one is stop reversal by ratcheting, this means is a type of mechanical absolute reversal stop and does not allow the polished rod rotate reversely, this means has a simple structure while the reliability is poor, at the same time, it also has the problems such as: the initial torque is too great and will cause a starting impact when restarts, and the elastic transformation must be released in workover operation, mechanical damage or human injury accidents will be caused when operation is not suitable. The second is stop reversal by hydraulic pressure, which has a high reliability, however, the cost is high and the structure is complex, thus the investment on apparatus is increased.